systemd/docs/ARCHITECTURE.md

title	category	layout	SPDX-License-Identifier
systemd Repository Architecture	Contributing	default	LGPL-2.1-or-later

The systemd Repository Architecture

Code Map

This document provides a high-level overview of the various components of the systemd repository.

Source Code

Directories in src/ provide the implementation of all daemons, libraries and command-line tools shipped by the project. There are many, and more are constantly added, so we will not enumerate them all here — the directory names are self-explanatory.

Shared Code

The code that is shared between components is split into a few directories, each with a different purpose:

• src/basic/ and src/fundamental/ — those directories contain code primitives that are used by all other code. src/fundamental/ is stricter, because it used for EFI and user-space code, while src/basic/ is only used for user-space code. The code in src/fundamental/ cannot depend on any other code in the tree, and src/basic/ can depend only on itself and src/fundamental/. For user-space, a static library is built from this code and linked statically in various places.

• src/libsystemd/ implements the libsystemd.so shared library (also available as static libsystemd.a). This code may use anything in src/basic/ or src/fundamental/.

• src/shared/ provides various utilities and code shared between other components that is exposed as the libsystemd-shared-<nnn>.so shared library.

The other subdirectories implement individual components. They may depend only on src/fundamental/ + src/basic/, or also on src/libsystemd/, or also on src/shared/.

You might wonder what kind of code belongs where. In general, the rule is that code should be linked as few times as possible, ideally only once. Thus code that is used by "higher-level" components (e.g. our binaries which are linked to libsystemd-shared-<nnn>.so), would go to a subdirectory specific to that component if it is only used there. If the code is to be shared between components, it'd go to src/shared/. Shared code that is used by multiple components that do not link to libsystemd-shared-<nnn>.so may live either in src/libsystemd/, src/basic/, or src/fundamental/. Any code that is used only for EFI goes under src/boot/efi/, and src/fundamental/ if is shared with non-EFI compoenents.

To summarize:

src/fundamental/

• may be used by all code in the tree
• may not use any code outside of src/fundamental/

src/basic/

• may be used by all code in the tree
• may not use any code outside of src/fundamental/ and src/basic/

src/libsystemd/

• may be used by all code in the tree that links to libsystem.so
• may not use any code outside of src/fundamental/, src/basic/, and src/libsystemd/

src/shared/

• may be used by all code in the tree, except for code in src/basic/, src/libsystemd/, src/nss-*, src/login/pam_systemd.*, and files under src/journal/ that end up in libjournal-client.a convenience library.
• may not use any code outside of src/fundamental/, src/basic/, src/libsystemd/, src/shared/

PID 1

Code located in src/core/ implements the main logic of the systemd system (and user) service manager.

BPF helpers written in C and used by PID 1 can be found under src/core/bpf/.

Implementing Unit Settings

The system and session manager supports a large number of unit settings. These can generally be configured in three ways:

1. Via textual, INI-style configuration files called unit files
2. Via D-Bus messages to the manager
3. Via the systemd-run and systemctl set-property commands

From a user's perspective, the third is a wrapper for the second. To implement a new unit setting, it is necessary to support all three input methods:

1. unit files are parsed in src/core/load-fragment.c, with many simple and fixed-type unit settings being parsed by common helpers, with the definition in the generator file src/core/load-fragment-gperf.gperf.in
2. D-Bus messages are defined and parsed in src/core/dbus-*.c
3. systemd-run and systemctl set-property do client-side parsing and translation into D-Bus messages in src/shared/bus-unit-util.c

So that they are exercised by the fuzzing CI, new unit settings should also be listed in the text files under test/fuzz/fuzz-unit-file/.

systemd-udev

Sources for the udev daemon and command-line tool (single binary) can be found under src/udev/.

Unit Tests

Source files found under src/test/ implement unit-level testing, mostly for modules found in src/basic/ and src/shared/, but not exclusively. Each test file is compiled in a standalone binary that can be run to exercise the corresponding module. While most of the tests can be run by any user, some require privileges, and will attempt to clearly log about what they need (mostly in the form of effective capabilities). These tests are self-contained, and generally safe to run on the host without side effects.

Ideally, every module in src/basic/ and src/shared/ should have a corresponding unit test under src/test/, exercising every helper function.

Fuzzing

Fuzzers are a type of unit tests that execute code on an externally-supplied input sample. Fuzzers are called fuzz-*. Fuzzers for src/basic/ and src/shared live under src/fuzz/, and those for other parts of the codebase should be located next to the code they test.

Files under test/fuzz/ contain input data for fuzzers, one subdirectory for each fuzzer. Some of the files are "seed corpora", i.e. files that contain lists of settings and input values intended to generate initial coverage, and other files are samples saved by the fuzzing engines when they find an issue.

When adding new input samples under test/fuzz/*/, please use some short-but-meaningful names. Names of meson tests include the input file name and output looks awkward if they are too long.

Fuzzers are invoked primarily in three ways: firstly, each fuzzer is compiled as a normal executable and executed for each of the input samples under test/fuzz/ as part of the test suite. Secondly, fuzzers may be instrumented with sanitizers and invoked as part of the test suite (if -Dfuzz-tests=true is configured). Thirdly, fuzzers are executed through fuzzing engines that try to find new "interesting" inputs through coverage feedback and massive parallelization; see the links for oss-fuzz in Code quality. For testing and debugging, fuzzers can be executed as any other program, including under valgrind or gdb.

Integration Tests

Sources in test/TEST-* implement system-level testing for executables, libraries and daemons that are shipped by the project. They require privileges to run, and are not safe to execute directly on a host. By default they will build an image and run the test under it via qemu or systemd-nspawn.

Most of those tests should be able to run via systemd-nspawn, which is orders-of-magnitude faster than qemu, but some tests require privileged operations like using dm-crypt or loopdev. They are clearly marked if that is the case.

See test/README.testsuite for more specific details.

hwdb

Rules built in the static hardware database shipped by the project can be found under hwdb.d/. Some of these files are updated automatically, some are filled by contributors.

Documentation

systemd.io

Markdown files found under docs/ are automatically published on the systemd.io website using Github Pages. A minimal unit test to ensure the formatting doesn't have errors is included in the meson test -C build/ github-pages run as part of the CI.

Man pages

Manpages for binaries and libraries, and the DBUS interfaces, can be found under man/ and should ideally be kept in sync with changes to the corresponding binaries and libraries.

Translations

Translations files for binaries and daemons, provided by volunteers, can be found under po/ in the usual format. They are kept up to date by contributors and by automated tools.

System Configuration files and presets

Presets (or templates from which they are generated) for various daemons and tools can be found under various directories such as factory/, modprobe.d/, network/, presets/, rules.d/, shell-completion/, sysctl.d/, sysusers.d/, tmpfiles.d/.

Utilities for Developers

tools/, coccinelle/, .github/, .semaphore/, .mkosi/ host various utilities and scripts that are used by maintainers and developers. They are not shipped or installed.

Service Manager Overview

The Service Manager takes configuration in the form of unit files, credentials, kernel command line options and D-Bus commands, and based on those manages the system and spawns other processes. It runs in system mode as PID1, and in user mode with one instance per user session.

When starting a unit requires forking a new process, configuration for the new process will be serialized and passed over to the new process, created via a posix_spawn() call. This is done in order to avoid excessive processing after a fork() but before an exec(), which is against glibc's best practices and can also result in a copy-on-write trap. The new process will start as the systemd-executor binary, which will deserialize the configuration and apply all the options (sandboxing, namespacing, cgroup, etc.) before exec'ing the configured executable.

 ┌──────┐posix_spawn() ┌───────────┐execve() ┌────────┐
 │ PID1 ├─────────────►│sd-executor├────────►│program │
 └──────┘  (memfd)     └───────────┘         └────────┘